I recently ruled at my table they act on their initiative, just to keep things simple. No changes in action economy or giving them possible extra actions, just an influence on what they do when things can be done.

You know, I've been interested in this line for years now. I never had a chance to really get into the Warhammer RPGs besides a few short-lived online games until 2014, when I introduced the game system to my primary gaming group. I decided it was awesome enough (and I had a shiny, new 2nd Edition Core book in my bag) to risk trying out given all of good reviews I'd heard, and none of my friends were into Warhammer so it was a clean slate for everyone to figure out things. The first game I ever GM'd was this system with that group (starting with the pre-made materials), and we're still playing with the occasional break to try out something different or let another person try out a storyline of theirs (I'm writing up a new campaign as we speak, actually).
Dark Heresy's ease of use has sold me on keeping it alive with the people I play with from now on, and the depth one can get out of it is great. I'm still quite sad that the license is falling out of Fantasy Flight Games' grasp, because I really think they did a stellar job by it. Sure, there's the occasional problem here or there; that happens everywhere, and there's always been Errata or a helpful local community to make sure things are taken care of.
I'm also taking inspiration from this; some friends and I have slowly been chipping together an updated, universal system for all of the branches of the FFG Warhammer games, and I've actually got some headway into my own personal project- updating Warhammer Fantasy to the 2e Ruleset. Someday, I want to hand my group my old copy of Trollslayer to give them a feel for the setting and then delve into it with no brakes.

Two things; I'd assume normal promethium and weapons-grade promethium are two different things, with additives designed to make them do their jobs better. Weapons-grade promethium probably has some kind of oxygenating agent mixed into it for use in atmospheres where oxygen is lacking, areas where one doesn't want to burn up all the air and for use in SPAAAAAACE. Lynata's point on how various regions use different formulas of promethium makes sense as well. I would also assume people put out promethium a bit like how they try to put out phosphorus and napalm; you literally scrape/rub it off, not just deny it fuel.

I actually curtailed Fate Point recovery in my game; it's not each session, but each day. It's a little more effort, sure. But it means when the players go all out in a short period of time, their resources are exhausted later on until they can get a break. It also prevents things like a pause mid-fight and full Fate Points when they return.

Xenos relics/remains they can trade to a Magos or Inquisitor for a real, non-heretical reward? There's definitely factions out there that'd want to study necrodermis and other Necron technology for whatever reason.

It is technically outside, but not far. It's also connected by transit systems. It makes sense, keeps the giant ships with lots of combustible materials from 'accidentally' blowing up and damaging the hive itself.
Also, I did a 'map' of Desoleum a while back; it was quite vague in all honesty, a side-view which I used to delineate between the various levels of the hive. It gave my players an idea of how classy the area they were entering was, which helped out plenty.

Yeah, this guy is effectively pretty nasty. It'll take 16 damage to get through his armour/toughness. From there, he regenerates 5 of that each turn. His weapon is above average, which means he'll probably scythe through your Acolytes if they are not careful/use cover adequately. The meltabomb is going to do some damage if they can manage to hit him with it, an average of 22 damage versus his 8 Toughness means 14 points gets through; given a few turns he could regenerate that back, however. If you max out the damage at 32, you end up doing 24 points of damage, which still doesn't kill him in one hit, or even put him into criticals by itself. Unless your group has weapons they can rely on to get through 16 Armour/Toughness (krak, plasma, melta, etc.), this is going to be a rather difficult fight.